Re: Preferred method for "Assignment by value"
On Apr 15, 3:51 pm, sturlamolden <[EMAIL PROTECTED]> wrote: > On Apr 15, 8:19 pm, [EMAIL PROTECTED] wrote: > > > Coming from VBA I have a tendency to think of everything as an > > array... > > Coding to much in Visual Basic, like Fortran 77, is bad for your mind. The distinction you're looking for is: VB: set a= collection a= collection Every assignment is a 'set'. -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
On Apr 15, 8:19 pm, [EMAIL PROTECTED] wrote: > Coming from VBA I have a tendency to think of everything as an > array... Coding to much in Visual Basic, like Fortran 77, is bad for your mind. -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
On Apr 15, 7:23 pm, [EMAIL PROTECTED] wrote:
> test = [[1],[2]]
> x = test[0]
Python names are pointer to values. Python behaves like Lisp - not
like Visual Basic or C#.
Here you make x point to the object which is currently pointed to by
the first element in the list test. If you now reassign test[0] = [2],
x is still pointing to [1].
> x[0] = 5
> test>>> [[5],[2]]
>
> x = 1
Here you reassign x to point to an int object vith value 1. In other
words, x is no longer pointing to the same object as the first element
in the list test.
That is why get this:
> test
>
> >>>[[5],[2]]
> x
> >>> 1
> test = [[1,2],[3,4]]
> I need to do some data manipulation with the first list in the above
> list without changing
> obviously x = test[0] will not work as any changes i make will alter
> the original...
You make a slice, e.g.
x = [from:until:stride]
Now x is pointing to a new list object, containing a subset of the
elements in list. If you reassign elements in x, test will still be
the same.
The point to remember, is that a list does not contain values, but
pointers to values. This can be very different from arrays in C, VB,
Java or C#:
a = [1,2,3,4,5] in Python
is different from
int a[] = {1,2,3,4,5};
The Python statement makes a list of five pointers, each pointing to
an immutable int object on the heap. The C statement allocates a
buffer of 5 ints on the stack.
If you can read C, the Python statement a = [1,2,3,4,5] is thus
similar to something like
int **a, i, amortize_padding=4;
a = malloc(5 * sizeof(int*) + amortize_padding*sizeof(int*));
for (i=0; i<5; i++) {
a[i] = malloc(sizeof(int));
*a[i] = i;
}
--
http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
[EMAIL PROTECTED] schrieb: > by changing temp = v[:] the code worked perfectly (although changing > temp.insert(0,k) to temp = [k] + temp also worked fine... I didn't > like that as I knew it was a workaround) So the for body now looks like this?: temp = v[:] temp.insert(0, k) finallist.append(temp) It can still be clarified and simplified to this (may also be faster): temp = [k] + v finallist.append(temp) Which one do you like better :)? Robin -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
[EMAIL PROTECTED] wrote: > Thank you both, the assigning using slicing works perfectly (as I'm > sure you knew it would)... It just didn't occur to me because it > seemed a little nonintuitive... The specific application was > > def dicttolist (inputdict): > finallist=[] > for k, v in inputdict.iteritems(): > temp = v > temp.insert(0,k) > finallist.append(temp) > > return finallist > Maybe, finallist = [[k] + v for k, v in inputdict.iteritems()] the list concatenation creating new lists. Duncan -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
I think the fundamental "disconnect" is this issue of mutability and immutability that people talk about (mainly regarding tuples and whether they should be thought of as static lists or not) Coming from VBA I have a tendency to think of everything as an array... So when I create the following test=[1,2],[3,4],[5,6] I'm annoyed to find out that I can change do the following test[1][1] = 3 but i can't do test[1] = [3,3] and so I throw tuples out the window and never use them again... The mental disconnect I had (until now) was that my original tuple was in affect "creating" 3 objects (the lists) within a 4th object (the tuple)... Previously, I'd been thinking of the tuple as one big object (mentally forcing them into the same brain space as multi-dimensional arrays in VBA) This was a nice "aha" moment for me... -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
http://effbot.org/zone/python-objects.htm still says it best. mt -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
Thank you both, the assigning using slicing works perfectly (as I'm sure you knew it would)... It just didn't occur to me because it seemed a little nonintuitive... The specific application was def dicttolist (inputdict): finallist=[] for k, v in inputdict.iteritems(): temp = v temp.insert(0,k) finallist.append(temp) return finallist to convert a dictionary to a list. We deal with large amounts of bankdata which the dictionary is perfect for since loan number is a perfect key... at the end, though, I have to throw it into a csv file and the csv writer doesn't like dictionaries (since the key is an iterable string it iterates over each value in the key) by changing temp = v[:] the code worked perfectly (although changing temp.insert(0,k) to temp = [k] + temp also worked fine... I didn't like that as I knew it was a workaround) Thanks again for the help -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
On Apr 15, 6:23 pm, [EMAIL PROTECTED] wrote: > As a relative new comer to Python, I haven't done a heck of a lot of > hacking around with it. I had my first run in with Python's quirky (to > me at least) tendency to assign by reference rather than by value (I'm > coming from a VBA world so that's the terminology I'm using). I was > surprised that these two cases behave so differently Perhaps it is better to think that you bind the name 'x' to the object '42' when you write 'x=42'. > test = [[1],[2]] > x = test[0] > x[0] = 5 > test>>> [[5],[2]] > > x = 1 > test > > >>>[[5],[2]] > x > >>> 1 > > Now I've done a little reading and I think I understand the problem... > My issue is, "What's the 'best practise' way of assigning just the > value of something to a new name?" > > i.e. > test = [[1,2],[3,4]] > I need to do some data manipulation with the first list in the above > list without changing > obviously x = test[0] will not work as any changes i make will alter > the original... > I found that I could do this: > x = [] + test[0] > > that gets me a "pure" (i.e. unconnected to test[0] ) list but that > concerned me as a bit kludgy > > Thanks for you time and help. To create a new list with the same elements as a sequence seq, you can use list(seq). 'list' is the type of lists, it is also a 'constructor' for list objects (the same goes for other common buit-in types, such as 'int', 'float', 'str', 'tuple', 'dict'). E.g. >>> foo = [1, 2, 3] >>> bar = list(foo) >>> foo[0] = 4 >>> foo [4, 2, 3] >>> foo = [1, 2, 3] >>> bar = list(foo) >>> bar[0] = 4 >>> bar [4, 2, 3] >>> foo [1, 2, 3] >>> HTH -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
On Apr 15, 10:23 am, [EMAIL PROTECTED] wrote: > As a relative new comer to Python, I haven't done a heck of a lot of > hacking around with it. I had my first run in with Python's quirky (to > me at least) tendency to assign by reference rather than by value (I'm > coming from a VBA world so that's the terminology I'm using). I was > surprised that these two cases behave so differently > > test = [[1],[2]] > x = test[0] > x[0] = 5 > test>>> [[5],[2]] > > x = 1 > test > > >>>[[5],[2]] > x > >>> 1 > > Now I've done a little reading and I think I understand the problem... > My issue is, "What's the 'best practise' way of assigning just the > value of something to a new name?" > > i.e. > test = [[1,2],[3,4]] > I need to do some data manipulation with the first list in the above > list without changing > obviously x = test[0] will not work as any changes i make will alter > the original... > I found that I could do this: > x = [] + test[0] > > that gets me a "pure" (i.e. unconnected to test[0] ) list but that > concerned me as a bit kludgy > > Thanks for you time and help. I think you understand the concept, basically you want to make a copy. Ether of these are acceptable: x = test[0][:] OR x = list(test[0]) this will also work: import copy x = copy(test[0]) Matt -- http://mail.python.org/mailman/listinfo/python-list
Re: Preferred method for "Assignment by value"
[EMAIL PROTECTED] wrote: > As a relative new comer to Python, I haven't done a heck of a lot of > hacking around with it. I had my first run in with Python's quirky (to > me at least) tendency to assign by reference rather than by value (I'm > coming from a VBA world so that's the terminology I'm using). I was > surprised that these two cases behave so differently > > test = [[1],[2]] > x = test[0] > x[0] = 5 > test > [[5],[2]] > x = 1 > test > [[5],[2]] > x > 1 > > Now I've done a little reading and I think I understand the problem... > My issue is, "What's the 'best practise' way of assigning just the > value of something to a new name?" > > i.e. > test = [[1,2],[3,4]] > I need to do some data manipulation with the first list in the above > list without changing > obviously x = test[0] will not work as any changes i make will alter > the original... > I found that I could do this: > x = [] + test[0] > > that gets me a "pure" (i.e. unconnected to test[0] ) list but that > concerned me as a bit kludgy > > Thanks for you time and help. > If you want a *copy* of an object (or portion thereof), you must explicitly *specify* a copy. THere are different ways of doing that depending on the object in question. Lists (and slices thereof) can be copied to a new list withthe slice syntax: L[1:4] will copy a limited portion, and L[:] will copy the whole list. The kind of copy is only one level deep -- the contents of the copy will be references to the contents of L Dictionaries have a copy method that creates a new dictionary. Again this copy is only one level deep. The copy modules provides a more general way to copy objects. It provides the ability to produce both shallow and deep copies. A small note: In a dozen years of programming in Python, I've used these various copy methods perhaps a dozen times or less. If you find you are copying structures often, you are probably not using Python as effectively as you could. Gary Herron -- http://mail.python.org/mailman/listinfo/python-list
Preferred method for "Assignment by value"
As a relative new comer to Python, I haven't done a heck of a lot of hacking around with it. I had my first run in with Python's quirky (to me at least) tendency to assign by reference rather than by value (I'm coming from a VBA world so that's the terminology I'm using). I was surprised that these two cases behave so differently test = [[1],[2]] x = test[0] x[0] = 5 test >>> [[5],[2]] x = 1 test >>>[[5],[2]] x >>> 1 Now I've done a little reading and I think I understand the problem... My issue is, "What's the 'best practise' way of assigning just the value of something to a new name?" i.e. test = [[1,2],[3,4]] I need to do some data manipulation with the first list in the above list without changing obviously x = test[0] will not work as any changes i make will alter the original... I found that I could do this: x = [] + test[0] that gets me a "pure" (i.e. unconnected to test[0] ) list but that concerned me as a bit kludgy Thanks for you time and help. -- http://mail.python.org/mailman/listinfo/python-list
